NEW YORK CITY—When Superstorm Sandy roared ashore with a surge of seawater in 2012, Sergej Mahnovski had been on the job directing the New York Mayor's Office of Long Term Planning and Sustainability for one week. He had a steep learning curve. In the wake of the storm surge 43 people were dead, Lower Manhattan lacked light at night and seven hospitals had to be evacuated. Post-Sandy, the long-term plan could quickly be reduced to two words: “never again.” It consisted of a range of major initiatives, from strengthening coastal defenses, whether seawalls or swamps, to ensuring food supplies in hospitals. "It's not just other storms but heat waves, heavy winds," Mahnovski told the audience at CityLab, a conference on urban solutions to global challenges sponsored by The Aspen Institute, Bloomberg Philanthropies and The Atlantic.

Cities are at the forefront of dealing with the impacts of global warming, so CityLab posed the question: Are they are also the best places to begin combating the pollution that causes climate change? As sociologist Daniel Bell once wrote, "The nation-state is becoming too small for the big problems of life, and too big for the small problems of life." So, in the same spirit, perhaps city governments offer solutions to the problems caused by the 90 million metric tons of greenhouse gases pumped into the atmosphere daily.

The world's cities add 65 million people per year, or the equivalent of more than 20 Chicagos or six New Yorks. India alone will add nearly 300 million urbanites by 2030, or a population roughly the size of the entire U.S. today. "We will spend more and build more in the next century than we have in all of human history before this," Richard Florida, an urban theorist at the University of Toronto, told the CityLab crowd on October 7. "Our cities have to be more environmentally sustainable."

That's because more than half the world's population of seven billion now live in cities and cities are responsible for 70 percent of greenhouse gas emissions globally. Roughly 75 percent of that pollution is under the direct control of city governments, whether municipal power plants or the like.

To begin to mitigate the pollution causing climate change, cities around the world need to be made more efficient, adopting measures ranging from reducing the energy costs of sanitation to constructing buildings that waste less energy. Former Vice Pres. Al Gore offered a list of recommendations at the conference, including the use of solar panels where feasible, trees and green roofs to combat the urban heat island, cisterns to deal with more intense but less frequent rainfall, and that metros "go whole-hog on electrification [of transportation]." Most importantly, Gore added, buildings must be made more efficient at heating, cooling and the like, a process that can be helped along by the revolution in information technologies, such as smart thermostats. "I see cities cutting greenhouse gas emissions and saving money in the process, but much more needs to be done," Gore said.

As Superstorm Sandy demonstrated by inflicting $19 billion in damages, cities will also have to adapt to a changed world. That may mean making politically unpalatable choices like retreat from current ocean shorelines, an effort actually under way in New York via buyouts of some coastal residents. "We haven't convinced voters that mitigation is important and we're going to spend billions on adaptation?" noted a skeptical Manny Diaz, former mayor of Miami, a coastal city even more at risk from the stronger hurricanes and sea level rise as a result of climate change.

Then there's the issue of growing metropolises and their impacts on climate. Suburban sprawl has helped U.S. emissions balloon in the last few decades. "Urban expansion on the scale the world is going to experience is going to require more land," argued economist Paul Romer of New York University’s Stern School of Business. "Any strategy based on containment is just doomed to failure."

But providing incentives for people to remain close to the metropolitan core, such as better public transportation, can help minimize the impact of city size on surrounding rural land. China is currently building 40 different metro lines in 40 different cities, each one covering roughly 1,000 kilometers, said Jonathan Woetzel, a director at consultancy McKinsey & Company's Cities Special Initiative. "One single act of infrastructure creates opportunities for new lives, new choices for 50 [million] to 100 million people" as well as cutting down on the demand for cars, roadways and their attendant pollution in the world's most populous country.

The challenge, as always, is how to pay—whether for metros, bus rapid transit or any other solution. "Cities are worth more than they cost to build so we should be able to get more of them at a profit," Romer noted, because the gain in the value of city property should be more than enough to compensate a government for building and running a city. After all, the roughly 1,200 square kilometers of land on which New York sits is now valued at around $600 billion, much more than the total cost of building the Big Apple. Or take London: "We're 23 percent of the country's [gross domestic product]," said Sir Edward Lister, that city's deputy mayor for policy and planning. Yet London does not garner 23 percent of the national government's investments.

One idea for getting around that comes from Hong Kong: the city government buys the land around any new transit line, and the sale of that now more valuable real estate funds the construction of the new line. "Their metro system is fundamentally a property company," Lister noted, adding that similar schemes financed the expansion of railroads in Europe and North America in the 19th century. "We have to return to that, that's what we lost. It's the same argument for electricity and gas where we've got the same problem."

Ultimately, strong local government would be required for any of these solutions. "A pervasive problem around the world is that city governments are not strong enough," Romer said, citing the example of New York City seizing land from private property owners in 1811 to construct the present street grid. "Very few city governments in the world can do what New York City did in 1811, and that's what is holding us back."

And global warming remains a global problem for which the nation-state remains indispensable even if city governments can act faster. "We have to put a price on carbon in the marketplace and we have to put a price on denial in the political system,” Gore argued.

Nevertheless, the megalopolis—or even gigalopolis—is likely to be the place where efforts to combat climate change, and adapt to it, actually happen. As sociologist Fran Tonkiss of the London School of Economics and Political Science put it: "Cities are just about the right size for dealing with some of the most serious problems we're facing."